The present invention relates to an improved press felt for use on a papermaking machine. More particularly, the invention relates to press felts that incorporate, as a component of the batt materials attached to one or both sides of the base fabrics of the felts, from about 20% to about 100% by weight of regenerated cellulosic staple fibers in at least one of the layers of batt material in order to improve the dewatering capability of the felt.
Press felts are endless belts which may contain a seam and which are used to convey an embryonic paper web from the forming section, through the press and into the dryer section of a papermaking machine so as to dewater and ultimately dry the paper product so that it is suitable for use. In the press section, at least one press nip is typically provided between either a pair of rotating cylindrical rollers, or a roller and concave shoe. The embryonic paper web passes through the at least one press nip laid either upon a single felt, or sandwiched between at least two press felts. As the web passes through the at least one press nip, water is expressed from it and passes into the at least one press felt.
Papermaker's press felts are well known. See, for example, U.S. Pat. No. 4,199,401 to Liu et al., U.S. Pat. No. 4,356,225 to Dufour, U.S. Pat. No. 4,414,263 to Miller et al., U.S. Pat. No. 4,806,413 to Penven, U.S. Pat. No. 5,360,656 to Rexfelt et al., and U.S. Pat. No. 5,864,931 to Best et al. These felts are usually comprised of a woven base fabric (typically formed of nylon or similar polymeric yarns) to which is attached, generally by needling, at least one layer of a pre-tacked staple fiber web, commonly referred to as a batt. Typical press felt batts will usually include between one and about 5 or more layers of a pre-tacked staple fiber web needled onto a first planar surface of the base fabric (usually the surface which, when in use, will be in contact with the paper sheet, and is hereafter referred to as the “PS”) to form a PS batt, and from none to one or more layers needled to the opposite planar surface (which when in use will be in contact with the equipment of the paper machine, and is hereafter referred to as the “MS”) to form the MS batt. The staple fibers used to form either or both the MS and PS batt are typically made from one or more nylons, polyesters or other polymeric materials such as are commonly employed in the manufacture of industrial textiles.
The batt provides a smooth surface for the paper web and a void volume into which water, which has been expressed from the paper web at the press nip, can be received. The base fabric provides some additional void volume, as well as a stable structure to which the batt can be attached. The base fabric is typically comprised of interwoven polymeric monofilament or multifilament yarns to which the batt is attached, generally by needling or other entanglement process such as is known in the art.
After the paper web has been pressed in at least one nip in the press section, it will still contain an appreciable amount of water, as much as from 30% to about 60% or more by weight. This remaining water must now be removed in the dryer section of the papermaking machine in order to provide a paper product. The final drying of the paper product is typically carried out by evaporative means, which requires a large amount of energy. This adds substantially to the cost of manufacturing the paper product. Generally, a 1% increase in the dryness of the sheet exiting the press section will translate into about a 4% energy savings in the dryer section. It is also possible that the speed of the paper machine may have to be reduced or at least limited due to the evaporative capacity of the dryer section.
Thus, it would be highly desirable if the water removal characteristics of the press felts could be improved so as to increase the amount of water they are capable of transporting away from the paper product as it passes through the press section.
It has been known to use regenerated cellulosics such as rayon as a component of papermaking fabric batt materials. However, such use has generally been restricted to certain specific circumstances. One know application provided an article of paper machine clothing for a press section of an impulse drying machine having a paper contacting surface layer which included a thermal barrier with sheet release properties, a base structure layer, and at least one intermediate layer. This intermediate layer could include fine denier fibers and/or hydrophilic fibers such as wool, cotton and regenerated cellulosics. Fabrics constructed in this manner and evaluated on a pilot scale impulse drying machine operating at 205° C. were reported to have achieved 4 to 5 percentage points of added dryness in the sheet. However, the intermediate layer was heat shielded, and the improved drying appears to have been mainly due to the high drying temperature of about 205° C. This was a press fabric for use at temperatures well above the normal operating temperature range of press sections, which typically run between about 40° C. and about 80° C., and clearly involved a different application.
Another known felt included a so-called “flow control” layer located between the batt and base to “impede rewetting of the paper web” as it exits the press nip. This flow control layer was reported to be formed of a spunbonded filamentary nylon material which is noncircular in cross-section (such as trilobal). It was also noted that the flow control layer could be formed from various materials, including rayon. However, a hydrophobic treatment was imparted to the flow control layer to prevent water absorption.
Another known press felt has been reported that includes a high proportion of fibrillatable fibers located in at least the PS surface of the batt so as to provide a relatively fine sheet supporting surface for the paper web. The PS surface was indicated as being formed from fibers which are as fine as possible (below 1 denier in size). These fine fibers occur as a result of the fibrillation of relatively larger regenerated cellulosic fibers (e.g. >1 denier in size) due to hydroentanglement or mechanical pressure.
A transfer fabric has also been known that includes a base structure and a fiber batt layer which is impregnated with a polymer matrix. The batt fibers differ from one another with respect to their surface properties so that the PS surface of the belt facing the web has both hydrophilic and hydrophobic areas.
The vast majority of press felts which are manufactured for, and are in use in paper mills today, consist of 100% nylon staple fiber in at least the batt, mainly due to its abrasion resistance, resiliency and tenacity.